I'm asking for a formula to calculate the $ difference (swing) between different $ scenarios.
A specific example is what is the swing between not renting a house (-$700/month) vs renting monthly (+$1000/month) for a swing of $1700. Other examples exist such as not renting (-$700) and renting on Airbnb (+200) for a $900 swing, or partial renting ($-700 vs -$100) for a $600 swing.
General examples of this are the difference between:
two negative numbers (eg., -$300 vs -$1000) = $700 swing
Mixed positive and negative (e.g., -$300 vs +$1000)= $1300 swing
two positive numbers (e.g., +$300 vs +1000)= $700 swing